Afghanistan braced itself Friday for a possible fourth day of violent anti-U.S. protests over the burning of Qurans at a NATO facility. At least 13 people have died in demonstrations since the protests erupted Tuesday.

U.S. President Barack Obama has sent a written apology to Afghan President Hamid Karzai about the desecration of the Islamic holy book at the Bagram airbase. He said “the error was inadvertent” and that officials will take “appropriate steps” to avoid any recurrence and hold those accountable responsible.

The U.S. embassy has urged Americans to avoid any unnecessary movement within the South Asian nation.

On Thursday, NATO officials said an individual wearing an Afghan army uniform shot and killed two coalition soldiers in eastern Nangarhar province during a protests over the Quran burning.

Earlier Thursday, the Taliban issued a statement calling on Afghans to launch attacks on foreign targets in retaliation for the desecration of the Muslim holy book.

But in a joint statement, the Afghan delegations assigned to probe the incident appealed to the Afghan people to “exercise self-restraint and extra vigilance” and avoid resorting to protests that may allow “the enemy to take advantage of the situation.”

Also Thursday, hundreds of protesters attacked a U.S.-run base in eastern Laghman province. In northern Baghlan province, officials said gunfire killed one protester and wounded at least two police officers.

On Wednesday, President Karzai appealed for calm, saying citizens have the right to protest, but should not resort to violence.

U.S. Deputy Defense Secretary Ashton Carter had met Wednesday in Kabul with Afghan leaders, including President Karzai, to apologize for the incident.

The commander of the international coalition, U.S. General John Allen, had issued an apology Tuesday, saying the improper disposal of Islamic religious texts was “not intentional in any way.” He ordered an investigation.